A PTC (Positive Temperature Coefficient) thermistor comprises at least a pair of electrodes disposed so as to face one another, and a thermistor element disposed between the pair of electrodes. The thermistor element has a “positive resistance-temperature characteristic” according to which the resistance value of the thermistor element increases rapidly with increasing temperature in a fixed temperature range.
A PTC thermistor employs this characteristic to protect the circuits of electronic devices such as self-regulating heaters, temperature sensors, current-limiting elements, overcurrent protection elements, and so on, for example. For use in these and other applications, demands are being made for PTC thermistors having a low room temperature resistance value when inoperative, a large rate of change between the room temperature resistance value when inoperative and the resistance value when operative, little variation in the resistance value when operated repeatedly (i.e. a small difference between the resistance value upon an initial stage of usage and the resistance value after repeated usage), an excellent cutoff characteristic, and an element with a low heat generation temperature. Demands are also being made for small size, light weight, and low cost.
A typical PTC thermistor is loaded with a thermistor element constituted by a ceramic material. However, this type of PTC thermistor has a poor cutoff characteristic and a thermistor element with a high heat generation temperature, and is difficult to reduce in size, weight, and cost. Particularly when used as an overcurrent protection element for a battery such as a lithium ion battery, the operating temperature of the PTC thermistor is preferably no more than 100° C., and more preferably between 80 and 95° C., but it is difficult for this type of PTC thermistor to satisfy such an operating temperature.
In response to such demands for a lower operating temperature and so on, a PTC thermistor (to be referred to where necessary as a “P-PTC thermistor” hereinafter) comprising a molded body constituted by a thermoplastic resin (high molecular matrix) and conductive particles as a thermistor element is under investigation.
An example of such a P-PTC thermistor, loaded with a thermistor element formed using low-density polyethylene as the high molecular matrix and nickel powder as the conductive particles (conductive filler) has been proposed (for example, in Japanese Unexamined Patent Application Publication H11-168005). This P-PTC thermistor is intended to have a comparatively low operating temperature (no more than 100° C., and preferably between 80 and 95° C.).